In the Third Age Isen and Adorn formed the boundary of the Kingdom of Rohan, but the triangle of land between Isen, Adorn, and the White Mountains was a contested land, claimed by the Rohirrim as well as the Dunlendings.[5]

The Isen formed a natural boundary in the Gap of Rohan, and was only crossable at Isengard or at the Fords of Isen, where the Rohirrim fought a number of great battles against the Dunlendings and Saruman's Orcs in the late Third Age.[6]

When the Ents attacked Isengard, they dammed the Isen and other springs and streams in Nan Curunír, and dug trenches to divert the river's flow, before breaking the dams and temporarily drowning all of Isengard.[7]